Decimals
The four operations on decimals, rounding to a chosen place, and converting between fractions and decimals.
⏱ 7 min · 🎯 4 things to master
You meet decimals every single day — the price at the hawker centre, your height in metres, the weight of fruit at the market. A decimal is just a neat way to write a fraction whose pieces are tenths, hundredths and thousandths. The golden rule for almost everything is to keep the decimal points lined up. Let's make decimals easy.
Parents: let your child predict before they reveal each step. The blue dotted words are tappable definitions, and the "Method tip" boxes name exactly what a PSLE marker rewards in the working — not just the final number.
By the end you'll be able to add and subtract decimals, multiply and divide them, round to a given place, and convert between fractions and decimals. Here we go.
Adding and subtracting decimals
When you add or subtract decimals, line up the so each digit sits in its correct place. Then add or subtract column by column, just like whole numbers, and bring the point straight down.
To add 3.40 and 1.25, stack them with the points aligned and add: 3.40 + 1.25 = 4.65. Adding a zero to make 3.4 into 3.40 keeps the columns tidy — it does not change the value but stops careless slips.
🤔 Predict first: What is 2.6 + 1.45?
Multiplying decimals
To multiply a decimal, first ignore the point and multiply as whole numbers. Then count the in the question and put the same number into your answer.
For 0.4 × 3, work 4 × 3 = 12, and since the question has one decimal place, the answer is 1.2. For 0.6 × 0.2, work 6 × 2 = 12, and since there are two decimal places in total, the answer is 0.12. Count the places carefully — that is the whole trick.
🤔 Predict first: What is 0.3 × 0.5?
Dividing decimals
To divide a decimal by a whole number, divide as usual and keep the decimal point in the answer directly above its place in the number. So 4.8 ÷ 4 = 1.2, with the point sitting above where it is in 4.8.
To divide BY a decimal, make the divisor a whole number first. For 3.6 ÷ 0.6, multiply both numbers by 10 to get 36 ÷ 6 = 6. Shifting both by the same amount keeps the answer the same but makes the working simple.
🤔 Predict first: What is 2.4 ÷ 0.4?
Rounding decimals
To a decimal, look at the digit just after the place you are rounding to. If it is 5 or more, round up; if it is less than 5, round down. To round 3.47 to one decimal place, look at the second decimal, 7, which is 5 or more, so 3.47 rounds up to 3.5.
To round 12.83 to the nearest whole number, look at the first decimal, 8, which rounds up, giving 13. Rounding helps you estimate and check money answers quickly.
🤔 Predict first: What is 5.62 rounded to one decimal place?
Converting fractions and decimals
A fraction and a decimal are two ways to write the same value. To turn a fraction into a decimal, divide the top by the bottom: three quarters means 3 ÷ 4 = 0.75. To turn a decimal into a fraction, write it over its place value and simplify: 0.6 is 6 tenths, which simplifies to three fifths.
Some fractions are worth memorising: one half is 0.5, one quarter is 0.25, three quarters is 0.75, and one fifth is 0.2. Knowing these by heart saves time in the exam.
🤔 Predict first: What is two fifths written as a decimal?
Watch out — these are easily mixed up
Quick recap
🎯 Mastery check
Answer all 6 — your progress is saved on this device.
What is 4.7 + 2.35?
What is 0.7 × 0.4?
What is 3.5 ÷ 0.5?
What is 8.46 rounded to one decimal place?
What is three fifths written as a decimal?
At a hawker stall, a drink costs $1.40 and a snack costs $2.65. How much change do you get from $5?